Genetically transmitted type C viruses of primates have been isolated from four baboon species and from animals of a related genus, Theropithecus gelada. Endogenous primate virogenes have been detected in all Old World monkeys, higher apes, and man, and viral antigens have been found in rhesus and patas monkey tissues and in several human tumors. Thus, both type C viral genes and their gene products can be detected in primate spcies from which infectious type C viruses have yet to be isolated. Viruses endogenous in primates have been transmitted to ancestors of the domestic cat and now code for a related group of genetically transmitted viurses (RD-114 group). An analysis of additional groups of type C viruses suggests that a second group of cat viruses (FeLV) were acquired from rats, while another group of viruses infectious for primates (SSAV/GALV) were derived from endogenous viruses of mice. The data lead to the conclusion that genetically transmitted type C viruses of one species can infect a second unrelated group of animals leading, in at least certain cases, to the widespread incorporation and perpetuation of acquired viral genes in the germ line of the recipient species.